In my aforesaid pending application, whose disclosure is incorporated herein by reference, there is disclosed a safety device for communication equipment, particularly a safety device which protects against the undesired application of overcurrent or overvoltage on the equipment side when the equipment is arranged between the output line side and the input line side in communication lines, and the overcurrent or the overvoltage on the output line side is produced by the output line contacting a high voltage line, or due to lightning. In the case of an overvoltage in my prior device, the facing bimetal protective elements formed a closed circuit when the temperature thereof increased because of a discharge, whereby the overvoltage was grounded. Accordingly, the overvoltage created no danger of a fire because the overvoltage protective elements had a potential difference of zero(0). Further, when the overcurrent and overvoltage were removed, the terminals were returned to their original state.
Within the prior art, FIG. 12, safety connector 100 is comprised of a cylindrical tube 105 which has a heating coil 102 wound on the outer circumference thereof. Tube 105 is secured on the upper surface of the connecting terminal that is mounted on the upper surface of the inserting terminal. As illustrated in FIG. 13, coil 102 is serially connected in the line of the safety connector.
In the upper end portion of the cylindrical tube 105 is inserted a projecting piece 104 which is secured to the tube 105 by solder 103 of relatively low melting temperature. On the upper surface of the projecting piece 104 is mounted a discharge tube 106 which is enclosed within an electrical ground case 101. As illustrated in FIGS. l2A and l3A, ground case 101 normally is physically and electrically out of contact with the connecting terminal. It is seen that each safety connector 100 serves two communication lines.
Spring 107 is retained in compression between ground case 101 and a ground plate 109, which is electrically and physical attached to ground terminal 108. When current below a predetermined value flows in the communication line heating coil 102, heating coil 102 of the safety connector 100 does not cause overheating and the solder 103 that holds projecting piece 104 above the end of tube 105 is not melted.
However, when current above a predetermined value flows through heating coil 102, this overcurrent causes heating coil 102 to produce sufficient heat to melt the solder that holds projecting piece 104 in the tube 105. Accordingly, projecting piece 104 is pushed downwardly and within the cylindrical tube 105 by the force of the spring 107 acting on ground case 101, discharge lamp 106 and the projecting piece. The circumferential surface on the lower end of ground case 101 now makes physical and electrical contact with the connecting terminal, see FIGS. l2B and l3B, and the overcurrent is coupled to ground through the path of the connecting terminal, ground case 101, spring 107, ground plate 109 and ground terminal 108. Accordingly, damage to the communication equipment by the overcurrent may be avoided.
Further, when overvoltage is applied in the circuit, a discharge is produced within the discharge tube 106 of the safety connector 100. Thus, the grounding circuit having the electrical path through the inserting terminal, tube 105, projecting piece 104, discharge tube 106, ground case 101, spring 107, ground plate 109 and the ground terminal 108 is formed to provide a path to ground. Accordingly, damage to the communication equipment by the overvoltage is avoided.
In the prior art device shown in FIGS. 12 and 13, the discharge tube 106 is a cylindrical ceramic member having two facing electrodes.
Because of the production of high heat in the prior art safety devices (for example, up to about 1,600.degree. C.), damage because of fire is possible.
Upon occurrence of the overcurrent as described above, the ground case 101 comes into contact with the connecting terminal. Accordingly, when the overcurrent or overvoltage is applied, the above-described prior art device has drawbacks which make it desirable to make changes in the safety connector 100, or components thereof. Further, the prior art device requires testing or inspection of each of the individual circuits because direct visual inspection of the safety connector 100 is not possible. Additionally, in the prior art safety device, when dealing with tens of thousands to hundreds of thousands of lines, the required size of the safety device becomes quite large.
Because the inner wiring of the terminal stand is connected by solder, the electric wire working processes are complex, difficult, and time consuming. Accordingly, the production cost is high. Additionally, errors in making the wiring connections reduce the reliability of such devices. Furthermore, Korean laid-open Utility Model Publication No. 2198/1983 (Publicated Nov. 14, 1983, "Safety Device for Communication") discloses constructions in which the safety circuit is returned to the original state, together with an overvoltage protective element that cut off overcurrent by resistance, and uses a base stand that includes printed circuitry.
In the above Utility Model Publication, by arranging printed circuitry on the base stand unit that provides the terminal stand, productivity is increased and wiring errors are eliminated. However, when each of the input and output terminals is, one by one, connected by solder joints on both end portions of the print circuit of the base stand unit, and in particular, when the overcurrent is caused by lightning, current above 200 A is instanteously passed. Accordingly, the printed circuitry possibly may be damaged. Further, because the overcurrent protective element that controls the flow of current is a resistance element, the fixed resistance value has a tendency to increase with time and usage. Thus, sensitivity of the communication equipment is lowered.
On the other hand, the overvoltage protective element is deficient in security because the discharge tube is possibly damaged by the heat of discharge.